The automatic erase feature, in which the typist need not key in an identification of the character to be erased (disclosed in the '846 patent), while considerably increasing keystroke efficiency, is, of course, only applicable where the line memory associated with the typewriter electronics includes identification of the character sought to be erased. In the event that the associated memory includes, at the print point at which the carriage is positioned, no erasable character, the typewriter leaves the erase mode and thus requires additional keystroking to effect the erasing action initially sought by depression of the erase key. In the event that the memory includes a code indicating a space function, the automatic erasing typewriter of the '846 patent will produce a backspace in response to the first erase depression and a second backspace to, in effect, search for an erasable character in the line memory, and when such character is found, it will be erased.
The first-mentioned patent teaches a manual erasing technique which serves to improve the keystroking efficiency over that available prior to that invention. This improvement required the operator to stroke a correction key with two effects; firstly, a backspace is performed to bring the print point and the type carrier into conjunction with the character sought to be erased, and the typewriter is conditioned such that on the next keystroke, rather than using the normal print ribbon, a special erase ribbon is interposed between the type carrier and the paper sheet when the type carrier next strikes the paper. Accordingly, when the operator next strokes a key, identical to the erroneously stroked key, the impression of that key is lifted off, or erased, from the paper. The operator can then strike the desired key to replace the erased character with the desired character.
In automatically erasing typewriters such as that disclosed in the '846 patent, the typewriter is only capable of automatically erasing a character when a code representing the character is retained in a (line) memory. Once the carriage is indexed (or carriage return is effected), the line memory is cleared and thereafter the automatic erase feature cannot be employed. Automatically erasing typewriters manufactured by IBM, however, are still capable of erasing a character even when (line) memory does not contain a representation of the erroneously typed character by simultaneously depressing the erase and "code" keys. This has the effect of backspacing the print point and on the next key depression, if a character key, the erase ribbon or medium is used to erase the character from the page. However, the operator must decide to use the code key when line memory is cleared in order to produce the desired result. This obviously places an added burden on the operator and will result in loss of keystroking efficiency if the operator's decision is incorrect.
The present invention seeks to improve operation of the automatically erasing typewriter in the event that the character sought to be erased is not contained in the line memory, and enables the automatically erasing typewriter to achieve the erasing function with a minimum amount of decision-making on the part of the operator. In accordance with the invention, the operator employs the same initial techniques regardless of whether or not the character sought to be erased is retained in the line memory. In particular, in accordance with the invention, the operator strokes the erase key which initiates the automatic erasing operation. In the event that the erroneously typed character is contained in the line memory, it is automatically erased as taught in the second referenced patent. In the event, however, that the erroneously typed character is not found in the line memory, signals are generated to condition the typewriter for manual erasure in accordance with the techniques identified in the first-mentioned patent. Thus, after stroking the erase key, in the event that the erroneously typed character is not found in line memory, the operator merely follows stroking the erase key by stroking the erroneously typed character key. Operation of the erase key in the event that the erroneously typed character is not found in the line memory, conditions the typewriter for manual erasure in which the operator must select the erroneously typed character so as to induce the erasure thereof.
Employing an automatic erasing typewriter of the present invention further improves the capabilities of such typewriter in other respects as well. In the automatically erasing typewriter disclosed in U.S. Pat. No. 3,780,846, when the operator strokes the erase key, and the typewriter finds a space function in the line memory, a second backspace is initiated, in effect, "looking" for an erasable character. Thus, if the typist is attempting to erase the "space", the automatically erasing typewriter requires the typist to either erase the immediately preceding character and begin typing at that point, or employ the backspace key rather than the erase key. In accordance with the present invention, when an erase operation is initiated, if the typewriter finds a space function in the associated line memory location, the backspace is terminated and the machine is conditioned for manual erasure. Operating the space bar then deconditions the typewriter from either an automatic or manual erase mode without causing an escapement to thereby enable the typist to continue typing to, in effect, "erase" the space by inserting a character therein.
Thus, in accordance with the invention, an automatically erasing typewriter which can type a sequence of characters on a page in response to operator actuation of a sequence of keys on the keyboard includes,
key actuated means for generating signals representing an actuated key,
memory means for storing an ordered sequence of signals representing a sequence of operator actuated keys,
print means including print and erase media for executing a print cycle to at times print a character if said print medium is enabled, or to erase a character if said erase medium is enabled,
print point position control means responsive to said key actuated means for displacing said print means and for accessing a selected location of said memory means corresponding to said print point, and
erase means operative in response to actuation of an erase key for initiating a sequence of operations including:
(a) actuating said print point position control means to displace said print point toward a left margin,
(b) accessing said memory means at a selected location corresponding to said print point for reading signals stored at said selected location,
(c) actuating said print means and enabling said erase medium to execute a print cycle to erase a character at said print point corresponding to signals read from said memory means, wherein the improvement comprises:
means responsive to absence of signals representing an erasable character at said selected location of said memory means to inhibit said print cycle and to enable said erase medium media and said printing means to execute a print cycle on a following key actuation only if said following key actuation corresponds to actuation of a key representing an erasable character.
More particularly, in accordance with the present invention, when an erase key is stroked, a backspace is initiated which has the effect of displacing the print point one character space rearwardly. Moving the print point has two effects; firstly, the type carrier or print means is displaced corresponding to moving of the print point, and secondly, the line memory location addressed is also correspondingly displaced. In the event that the line memory contains a character code, a print cycle is initiated with the erase medium enabled and the automatic erase functon is carried out. In the event, however, that the line memory does not contain a character code, the typewriter print cycle is inhibited and instead the typewriter is conditioned to manually erase, in which, on the next keystroke, comprising an erasable character, manual erase is effected by initiating a print cycle with the erase medium enabled, inhibiting or preventing escapement of the print means and finally, deconditioning the typewriter from an erase mode. In the event that the operator strokes a key other than an erasable character, the typewriter either reinitiates the automatic erase sequence in the event that the subsequently struck key is the erase key, or merely deconditions the typewriter from an erase mode if the subsequently struck key is the space bar.